Many of the firearms used to kill thousands of police and government officials in Mexico come from gun shops and gun shows in Southwest border states, the report says.

The report acknowledges that there are significant gaps in the data. It also blames both US and Mexican authorities for the problem, citing US laws that allow "paperless" sales of firearms between private individuals and corruption in Mexico that makes it difficult for the US to help it fight the arms influx.

The effort by some Congressional Democrats, as well as Attorney General Eric Holder and Secretary of State Hillary Clinton, to bring the issue to light, says Vizzard, is largely to placate Mexican counterparts, who are seeing a burst of violence as police attempt to tamp down drug cartels. He adds that it's also an opportunity under a Democratic US president to make an international point about America's role in the Mexico bloodshed.

"Washington has to pacify the Mexican government, and, rightfully, the Mexican government is pointing at the US saying, 'You guys keep talking about our drugs going to the US. What about your guns coming down here?' " says Vizzard, adding, "And they legitimately have a beef."

After the landmark Heller decision last year – in which the Supreme Court affirmed Second Amendment gun rights – Democratic leadership has stepped back from pressing the gun control issue, at least at the national level. While popular in urban centers, gun control laws can have electoral implications in rural hunting states where Democrats made huge gains last election.

If anything, gun rights have expanded on the heels of last year's Heller decision. The Democrat-dominated Congress this year agreed to allow Americans to carry concealed weapons into national parks and wildlife refuges.

But the Brady Center to Prevent Gun Violence hopes that today's Congressional hearings on the GAO report will have some effect on efforts to close the so-called "gun show loophole" where guns can be sold without background checks.

"The extremist gun lobby should no longer be permitted to dictate our nation's gun policy," said Brady Center president Paul Helmke, in a statement.

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